Daniel Moye's Technical Writings

Engineering Geology for the Snowy Mountains Scheme

No works come within the small area affected by this third
glacial stage, The effects of the older second and third glaciations
have been much modified by normal river erosion and denudation
in the period following the glaciations, particularly at the lower
levels where the streams are now entrenched well below their former
glaciated floors. The glaciations generally, therefore, do not have a
direct effect on many works. One project, however, the proposed
Kosciusko Dam on Spencer’s Creek, a tributary of the Upper
Snowy River, is greatly affected.

Rock Weathering:

Much of the undulating plateau country, the broad mature
valleys and even most of the higher slopes of the steep-walled
youthful valleys entrenched below the plateau surface, such as those
of the Tumut and Upper Snowy Rivers, are covered with a mantle
of residual soil often of great thickness formed by the weathering
in place of the bed-rock. Only along the beds of actively degrading
streams and the lowest hundred feet or so of the major valleys is it
common to find fresh rock exposed at the surface.

Determination of the extent of the weathering and the nature
and engineering properties of the products of weathering are among
the most frequent and important geological problems encountered.
They arise in connection with the investigation of the sites of
practically every dam, tunnel and power station. Weathering often
severely limits the number of possible quarry sites for concrete
materials, rock fill and rip-rap. Residual soils often provide the
most suitable and abundant sources of earth-fill for dam embankments
and road building materials. Weathering greatly affects the
location and design of roads in steep country.

Apart from these direct effects on the engineering works,
the widespread blanket of soil makes geological mapping difficult by
concealing the underlying rock and, in particular, it often tends to
obscure weak-zones in the bed-rock, since both sound rock and weak
rock are reduced by weathering to a similar soil.

Weathering is by no means uniformly developed throughout
the area. The most important factors influencing its development
are the rock type, the local geological structure and the erosional
history of each particular locality.

The granitic rocks are the most abundant and widespread
rocks in the area, and it was therefore necessary at an early stage in
the investigation to closely define the terms used to describe the
various degrees of weathering of granitic rocks in order that descriptions
of weathered rock exposed in outcrops, excavations, or in drill
cores made by different geologists would be consistent and would
indicate the engineering properties of the materials. Weathering
of granite usually follows a regular pattern of development, and the
degree of weathering can be consistently judged. Weathering in
this case is mainly chemical weathering, recognized by the softening
and decomposition of the felspars to clays, decay of the biotite
mica, frequent brown discoloration throughout the fabric of the
rock due to limonite, and a tendency for the rock to break into individual
mineral grains, all leading to a reduction in its strength
compared with its fresh state.